Category Tallest Buildings

The Burj Khalifa

If you happen to check in to the Grand Hyatt San Francisco on a windy day, you’ll receive a friendly note at the front desk advising you that the 35-story skyscraper may creak a bit as it moves gently back and forth in the wind. Though the hotel assures guests that this quirk is not an indication of any structural problem, the issue has nevertheless prompted complaints from visitors.

“The building CREAKS!” exclaims one exasperated and sleepless customer in his review of the hotel.1 “It sounds like you’re on an old ship,” writes another.2

The Burj Khalifa, formerly the Burj Dubai, is the world's tallest buildingFrom the disconcerting to the dangerous, wind has always been an important consideration when constructing skyscrapers. Since the 10-story steel-frame Home Insurance Building, the world’s first skyscraper, opened in Chicago in 1885, architects have had to think about wind stress, or “wind loading,” as they’ve built higher and higher.3 Today, wind engineering is an integral aspect in the design of any new tall building, especially the very tallest of them all: the Burj Khalifa.

At 2,717 feet, the Burj Khalifa, formerly known as the Burj Dubai, rises like a bolt of lightening into the sky, dwarfing the surrounding skyscrapers. The tower, which opened on January 4th, became the world’s tallest building, outdoing the previous record-holder, the Taipei 101, by a staggering 1,046 feet. (The Burj is about as tall as the Taipei 101 with the Chrysler building stacked on top.) Over half a mile from the base to the tip of its spire, the tower redefines the term “supertall,” a name often applied to skyscrapers over 1,000 feet.

The Burj Khalifa is specially designed to conquer the wind, a goal that becomes more and more important as altitude increases. The building rises to the heavens in several separate stalks, which top out unevenly around the central spire. This somewhat odd-looking design deflects the wind around the structure and prevents it from forming organized whirlpools of air current, or vortices, that would rock the tower from side to side and could even damage the building. Even with this strategic design, the 206-story Burj Khalifa will still sway slowly back and forth by about 2 meters at the very top.

The Burj Khalifa’s talent for “confusing the wind,” as chief structural engineer Bill Baker calls it, is just one of the methods used to help supertalls resist wind stress.4 Over four thousand miles away near the coast of Taiwan, stands the Taipei 101 tower, now a distant second at 1,667 feet. Inside, between the 88th and 92nd stories, a giant pendulum, known as a tuned mass damper, does quiet battle with deadly windstorms and typhoons. The gold-colored, 730-ton orb swings gently back and forth, balancing the tower against the forces of the wind and ensuring the comfort of its occupants.5

The tuned mass damper, also used in Boston’s John Hancock Building and New York City’s Citigroup Center, is a commonly employed mechanism for reducing the wind’s action on a skyscraper. The size and shape of the damper is “tuned” based on the height and mass of each particular tower. As the wind pushes the building in one direction, the damper swings or slides the other way, reducing sway similar to the way shock absorbers on a car soften bumps in the road. “You’re adding a component to the building that’s going to take the motion rather than the building itself,” explains Jason Garber, a wind-engineering specialist at RWDI, a leading wind tunnel testing firm.6

When constructing a skyscraper, consideration of the wind is paramount, says Carol Willis, director and curator of the Skyscraper Museum in New York.7 Throughout the design process, structural engineers and wind specialists work meticulously to alleviate wind stress, ensure structural stability and guarantee the comfort of occupants. Using both structural solutions, such as the Burj Khalifa’s method of “confusing the wind,” and mechanical ones, such as the tuned mass damper, designers do constant battle against the tireless wind.

The Burj Khalifa, says Bill Baker, is like a Swiss watch, every part working together to “resist the forces of nature such as wind, seismic and gravity.”  Yet forces like gravity are comparatively simple to deal with. Gravitational forces pull the skyscraper in only one, quite predictable, direction: down. But high-altitude winds swirl and jostle in complex and uncertain ways, whipping into eddies and vortices that put all different kinds of stress on the structure.

As Garber explains it, a building is like “a giant sail” with a great deal of area that the wind can push against. “The wind is blowing on the building causing it to sway and twist,” he says. “For certain shapes, the wind can form a wake similar to what you’d see behind a boat with vortices shedding off, alternating on either side and pushing the building from side to side.”8

“This causes a regular, or periodic, force,” continues Garber, “that pushes the building side to side across from the wind flow. The frequency at which that happens will vary with wind speed and if those vortices can align with the frequency that the building wants to oscillate at then you can get some very larges forces developed.”

Like a guitar string, buildings have a natural, or resonant, frequency at which they are inclined to vibrate. Wind vortices will only have a significant effect on a building when their frequency lines up just right, just as an opera singer has to hit the perfect pitch to shatter a wine glass. If by chance the vortices happen to push back and forth at the same rate as the structure’s resonant frequency, they can generate huge forces, as was the case in the Tacoma Narrows Bridge collapse in 1940. As a result of this effect, a key goal in skyscraper design is to disrupt the organized flow of wind around the building.

“What they’ve done on the Burj Khalifa deliberately,” explains Garber, “is keep introducing changes to the shape of the building with height so that the flow pattern can’t organize itself. It’s almost like you have several different buildings with height and each one of them has different vortices shedding at different winds speeds. All of those things can’t happen at the same time so what you’re left with is very little vortex shedding.”

If not properly addressed, wind stress from vortex shedding could theoretically cause major structural damage or even collapse. No need for queasiness though, as today’s skyscrapers are strong enough to withstand the most extreme winds speeds, making true structural failure a near impossibility. Skyscrapers are engineered according to a 50- or 100-year return rule, meaning that, on average, engineers expect winds to reach structurally dangerous speeds only once in a half century or more. Just to be safe, designers then increase the strength of the structure by an additional 60% or so to account for uncertainty in their measurements. When all this is taken into account, says Garber, “you’re talking about something along the lines of a 500- or 1000-year event.” The bottom line, he says, is that these buildings aren’t in any risk of falling over.9

Still, wind stress can still cause all kinds of problems in tall buildings. It can break windowpanes, damage the outer façade, stress building joints, cause leaking, crack walls and create myriad other issues. In addition, it can result in unnerving, even nauseating, swaying.10

“If the building’s moving too much, sometimes you can hear it creaking,” says Garber. However, “the most common concerns are of excessive motions. You might get people complaining that they feel the building moving or they might even feel sick.” Such was the case in the former Gulf & Western building in New York City. As a result of wind stress, the 44-story building developed cracks in stairwells and interior walls. In addition, office workers on the upper floors frequently complained of motion sickness on windy days. To fix these problems, owners invested over $10 million to add a massive steel brace to steady the structure.11

Indeed, measures to counteract the wind are undertaken as much for comfort as for safety. The happiness of occupants is an especially important issue to structural engineers, says Willis. “People are more sensitized than structures are to wind. Tuned mass dampers, for example, are used to address acceleration and peoples’ queasiness and response to the sway of buildings.”

Wind stresses grow dramatically the higher you build. Not only do wind speeds increase with height, but the force of the wind also increases with the square of its velocity. That means rapidly growing wind stress as the height of the building increases, which can cause even the most rigid skyscrapers to sway slowly back and forth.

“In any building,” says Garber, “the amount of motion you’d expect is on the order of 1/200 to 1/500 times its height.” For the Burj Khalifa, this translates into about two to four meters. “It’s not much, but certainly enough to make residents queasy if they can sense this motion. That’s why one of the chief concerns of architects and engineers is acceleration, which can result in perceptible forces on the human body.”

In carnival rides, cars and planes, physicists often think about forces in terms of “g’s,” multiples of the force of gravity. “When we are looking at buildings,” explains Garber, “we’re talking about milli-g’s of force.” As long as the occupants can’t feel the building moving, a certain degree of sway is acceptable and even expected. Humans can sense acceleration as small as about 5 to 25 milli-g’s, far less than what the structure can actually withstand.12 In most cases, such as the John Hancock building and Taipei 101, tuned mass dampers are installed not to ensure structural stability but to prevent queasiness.

Skyscrapers undergo rigorous wind tunnel testing during the initial design phase. Rowan Williams Davis & Irwin Inc. (RWDI), one of the world’s leading wind engineering consulting firm, has handled the testing for numerous projects around the world including the Burj Khalifa and Taipei 101.

At RWDI, wind-engineering experts collaborate with the building’s structural engineers early on. Prior to construction, wind-engineering specialists are given complete architectural drawings of the building and the team at RWDI then gets to work constructing a complex, rigid scale model for testing. These models are covered in small holes, called pressure taps, used to measure the effects of the wind. The 1:500 scale model of the Burj Khalifa, for example, contains 1,140 separate pressure taps for collecting wind data.13

These elaborate replicas go through several rounds of testing in a specialized wind tunnel. Unlike the tunnels used to test airplane wings, sporting equipment and other small projects, these boundary layer wind tunnels are designed to simulate changes in the wind speed with height and can replicate the variable wind environments in which the buildings will ultimately be constructed. Inside the tunnel, the model is rotated at all different angles and wind effects are sometimes visualized using smoke. All of this data is then fed into computer models in order to perform additional analysis. In the case of the Burj Khalifa, wind tunnel testing led to a dramatic design change: the entire building was rotated 120º to reduce wind loading. Ultimately, this process of wind testing, provides structural engineers with a nuanced understanding of wind loads.

Tallest buildings in the whole world

This list of the world’s tallest buildings includes only those with continuously occupiable floors, as opposed to non-building structures such as TV towers. Roof or spire height is taken into consideration in this ranking, but not antenna height.

1. Burj Khalifa, Dubai, UAE. (2010), 828 m, 163 floors.

Has held the title of world’s tallest building since 2010.

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2. Shanghai Tower, Shanghai, China (2015), 632 m, 121 floors

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3. Makkah Clock Royal Tower Hotel, Mecca, Saudi Arabia (2012), 601 m, 120 floors

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4. Ping An Finance Centre, Shenzhen, China (2017), 599 m, 115 floors

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5. Lotte World Tower, Seoul, South Korea (2017), 554.5 m, 123 floors

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6. One World Trade Center, New York City, USA (2014), 541.3 m, 104 floors

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7. Guangzhou CTF Finance Centre, Guangzhou, China (2016), 530 m, 111 floors

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8. Taipei 101, Taipei, Taiwan (2004), 508 m, 101 floors

World’s tallest building 2004 – 2010.

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9. Shanghai World Financial Center, Shanghai, China (2008), 492 m, 101 floors

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10. International Commerce Center, Hong Kong, China (2010), 484 m, 108 floors

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11. Lakhta Center, St. Petersburg, Russia (2018), 462 m , 86 floors.

12. Petronas Towers, Kuala Lumper, Malaysia (1998), 451.9 m, 88 floors. World’s tallest building 1998 – 2004.

13. Zifeng Tower, Nanjing, China (2010), 450 m, 89 floors

14. Willis Tower, Chicago, USA (1974), 442.1 m, 108 floors. World’s tallest building 1974 – 1998.

15. KK100, Shenzhen, China (2011), 441.8 m, 100 floors

16. Guangzhou International Finance Center, Guangzhou, China (2010), 437.5 m, 103 floors

17. 432 Park Avenue, New York (2015), 425.5 m, 85 floors

18. Trump International Hotel & Tower, Chicago, USA (2009), 423.4 m, 98 floors

19. Jin Mao Tower, Shanghai, China (1999), 420.5 m, 93 floors

20. Princess Tower, Dubai, UAE (2012), 414 m, 101 floors

21. Al Hamra Tower, Kuwait City, Kuwait (2011), 412.6 m, 90 floors

22. Two International Finance Centre, Hong Kong, China (2003), 412 m, 90 floors

23. 23 Marina, Dubai, UAE (2012), 392.8 m, 89 floors

24. CITIC Plaza, Guangzhou, China (1996), 391.1 m, 80 floors

25. Shun Hing Square, Shenzhen, China (1996), 384 m, 69 floors

26. Eton Place Dalian Tower 1, Dalian, China (2015), 383.1 m, 81 floors

27. Burj Mohammed Bin Rashid, Abu Dhabi, UAE (2014), 381 m, 88 floors

28. Empire State Building, New York City, USA (1931), 381 m, 102 floors. World’s tallest building 1931 – 1970.

29. Elite Residence, Dubai, UAE (2012), 381 m, 91 floors

30. Central Plaza, Hong Kong, China (1992), 374 m, 78 floors

31. Bank of China Tower, Hong Kong, China (1990), 367.4 m, 72 floors

32. Bank of America Tower, New York City, USA (2009),365.8 m, 55 floors

33. Almas Tower, Dubai, UAE (2009), 360 m, 68 floors

34. JW Marriott Marquis Dubai 1 & 2, Dubai, UAE (2012/2013), 355.4 m, 77 floors

35. Emirates Office Tower, Dubai, UAE (2000), 354.6 m, 56 floors

36. OKO – Residential Tower, Moscow, Russia (2015), 354.1 m, 85 floors

37. The Torch, Dubai, UAE (2011), 352 m, 86 floors

38. T & C Tower, Kaohsiung, Taiwan (1997), 347.5 m, 85 floors

39. Aon Center, Chicago, USA (1973), 346.3 m, 83 floors

40. The Center, Hong Kong, China (1998), 346 m, 73 floors

41. John Hancock Center, Chicago, USA (1969), 343.7 m, 100 floors

42, ADNOC Headquarters, Abu Dhabi, UAE (2015), 342 m, 65 floors

43. Chongqing World Financial Center, Chongqing, China (2015), 339 m, 73 floors

44. The Wharf Times Square, Wuxi JS, China (2014), 339 m, 68 floors

45. Mercury City Tower, Moscow, Russia (2013), 338.9 m, 75 floors

46. Tianjin Modern City Office Tower, Tianjin, China (2016), 338 m, 65 floors

47. Tianjin World Financial Center, Tianjin, China (2011), 336.9 m, 76 floors

48. Shanghai Shimao International Plaza, Shanghai, China (2005), 333.3 m, 60 floors

49. Rose Rayhaan, Dubai, UAE (2007), 333 m, 72 floors

50. Minsheng Bank Building, Wuhan, China (2008), 331.3 m, 68 floors

Competition for Tallest Building

With the latest generation of high-rise buildings reaching new heights of close to 2,000 feet, the supertall construction boom is bringing new challenges as projects are built higher, faster and with increasing complexity. As the (re)insurer for many of the world’s tallest buildings, including the soon to be ‘world’s tallest building’, the 3,280 ft. Kingdom Tower in Jeddah, and New York’s One World Trade Center, engineering insurer Allianz Global Corporate & Specialty (AGCS) analyzes the challenges of assessing and managing such exceptional risks in its latest Supertall Buildings Risk Bulletin.

The growth of the world’s tallest buildings continues to accelerate in the 21st century. By 2020, the average total height of the tallest 20 buildings in the world is expected to be close to 2,000 ft., or double the height of the Eiffel Tower, made possible by a combination of new technologies, innovative building materials and creative design elements.

US skyscraper dominance is declining. North America now accounts for only 16% of the world’s tallest buildings. At 1,776 ft., One World Trade Center is just over half the proposed height of Kingdom Tower but is the tallest building in the Western Hemisphere and fourth largest in the world.

South East Asia (48 percent) and the Middle East (30 percent) are home to more than three quarters of the tallest 100 buildings. China has 30 of the world’s top 100 tallest buildings in 15 cities, double North America. Dubai is home to 20 percent of the tallest 50 buildings.

“The eastward trend is set to stay, driven by rapid economic and demographic growth, urbanization, strong investor appetite for flagship real estate assets and lower labor costs than in the traditional Western markets,“ explains Ahmet Batmaz, Global Head of Engineering Risk Consulting at AGCS.

Elevators biggest obstacle to first mile high building

While concepts for the first mile-high building already exist, elevator technology is lagging behind building technology. Elevators currently can only transport people about 2,000 ft., mainly due to challenges in braking and cabling technology. Other limiting factors include:

  • Availability of building materials to potentially replace steel and cement;
  • Safety measures for occupants and surrounding areas;
  • Damping systems to reduce the negative impact from wind or seismic activity;
  • Financing for these mega projects.

At the same time new challenges continue to arise. Glass facades have raised concerns about the long-term impact of “solar gain” – the extent to which a building absorbs sunlight and heat – with governments introducing regulations around shape and structure.

Insuring billion dollar buildings

AGCS engineering risk experts maintain that these projects are highly complex, as they can involve up to 10,000 workers and over 100 subcontractors. Potential challenges of supertall constructions include:

  • Building material choice: Glass panels need to be thicker and more durable for the higher stories, while concrete mixes must vary to withstand the differing building loads that vary with height;
  • Cranage and lifting items to extreme heights;
  • Significant variations in wind speeds between ground and upper levels;
  • Maintaining verticality as the building height increases;
  • Elastic shortening of constructed building elements as the imposed weight from the completed building increases;
  • Fire risk both during construction and occupied phases – Efficient evacuation of a building which has multiple purposes like hotels, restaurants, residential areas, shopping centers and offices is crucial.